The present invention relates to well logging tools, and more particularly to electromagnetic logging methods and apparatus for measuring properties of earth formations penetrated by a borehole. More particularly, the present invention relates to an electromagnetic dipmeter method and apparatus for measuring the dip and strike of formations relative to the borehole.
The basic principles and techniques for electromagnetic logging for earth formations are well known. Induction logging to determine the resistivity (or its inverse, conductivity) of earth formations adjacent a borehole, for example, has long been a standard and important technique in the search for and recovery of subterranean petroleum deposits. In brief, the measurements are made by inducing eddy currents to flow in the formations in response to an AC transmitter signal, and then measuring the appropriate characteristics of a receiver signal generated by the formation eddy currents. The formation properties identified by these signals are then recorded in a log at the surface as a function of the depth of the tool in the borehole.
One significant limitation of current electromagnetic logging technology has been the lack of a commerically viable regional dipmeter suitable for use in dipping formations. There have been theoretical studies, but practical solutions have not yet been found. One approach, for example, was not considered practical because of its high vulnerability to borehole effects such as caused by the borehole fluid (see J. H. Moran and S. Gianzero, "Electricall Anisotropy: Its Effect on Well Logs", Developments in Geophysical Exploration Methods, Vol. 3, Chapter 6, Applied Science Publishers, 1982, pp. 195-238, at p. 231). The Moran and Gianzero solution was otherwise quite straightforward, benefiting nicely from the simplifications afforded by using a zero spacing between the transmitter and receiver coils. Unfortunately, it was this same zero spacing which, while making it possible to specify an analytical solution for such a dipmeter, also caused the intense vulnerability to borehole effects. Adding to these limitations, zero spacing also meant that any eccentering signal proved to be much, much greater than the formation signal.
A need therefore remains for an electromagnetic dipmeter method and apparatus for measuring the dip and strike of formations relative to a borehole. More particularly, a need remains for such a dipmeter which can accurately measure the dip and strike without appreciable vulnerability to borehole effects or physical effects such as tool eccentering. Preferably, such a dipmeter will be able to analyze and determine such dip and strike information accurately, rapidly, and reliably, and without excessively complicated theoretical and/or computational procedures, so that it can afford widespread practical application and utilization in a great variety of geophysical conditions and environments.